Using mod_perl *aside* from deploying web apps?

It is unfortunate because: first of all, mod_perl is much more than just about running web apps, it is actually a tool to customize and extend Apache using Perl instead of C. There's so much you can do with it.

Second, I believe mod_perl is suboptimal for running web apps: it makes your Apache processes fat (and thus you'll often need a front-end proxy), it's tied to Apache (so you can't experiment with other webservers), it's relatively more complex to configure and tune compared to, say, FastCGI (and thus potentially more insecure), and it's just too damn powerful if you just want a fast CGI.

mod_perl is also theoretically more insecure because it bundles webserver and the application engine together instead of cleanly separates it. Some of your Perl code might run as root too. All of these are unnecessary if you just want to run web apps.

Here at work we have been using mod_perl for years, not for deploying web apps but for creating custom Apache handlers in Perl (basically it was because my C sucks). The handlers do the following:

connect to CGI daemon (because we also write our own CGI daemon, which is much more paranoid in some respects but also more flexible in others like running PHP scripts under different configurations);

filter URLs using own rules (this can be done using a series of regexps with mod_rewrite, but much more readable and comfortable if done with full-fledged Perl code);

authenticate hosting users;

do per-vhost aliases (mod_alias can also do this, but we are using mass/dynamic virtual hosts);

etc.

The servers on which we run mod_perl are shared hosting servers, and we allow users to install their own .htaccess, so we have to patch mod_perl to restrict the Perl* directives from the users. This is because mod_perl does not have something like mod_ruby's RubyRestrictDirectives. This kind of functionality is available as a build-time configuration.

So there you have it, an unusual/unpopular application of mod_perl: customizing Apache in a shared hosting environment.

Sadly, it's just a very quick-n-dirty and crude hack, not a proper patch. I'd probably try to submit a more proper patch if there is someone like me who needs something similar. But so far haven't found one, apparently no one is weird (stupid?) enough to run mod_perl on a shared hosting server :-)